The present invention relates to a sealed alkaline storage battery having an electrode group, and more particularly to an improvement of separators used therein.
Heretofore, a sealed alkaline storage battery has adopted the Neumann effect to suppress an increase in an internal pressure of the battery. According to this effect, oxygen gas generated at positive electrodes in overcharging is absorbed at negative electrodes. However, if an oxygen gas absorption efficiency of the negative electrodes is low, the internal pressure of the battery increases gradually. Such an increase in internal pressure leads to breakdown or shortened life of the battery. Therefore, investigations have been made in various ways to improve the oxygen absorption efficiency of the negative electrodes.
For example, Japanese Laid-Open Patent Publication Hei 6-20673 proposes a method in which water repellency is imparted to a separator on the surface facing a positive electrode. But, this method adds a step of giving the water repellency to the separator. Therefore, a process to produce a battery is complicated and a production cost must be increased. Another problem is that giving the water repellency reduces an amount of an electrolyte which contacts with the electrode, leading to a decrease in discharge characteristics of the battery, especially high-rate discharge characteristics.
Further, Japanese Laid-Open Patent Publication Hei 8-111232 proposes a separator in which density of the inner layer is lower than that of the outer layers. However, such separator provides a decrease in oxygen gas absorption capacity of a negative electrode since the electrolyte reserved on the surface of the negative electrode becomes relatively large and the area in contact with oxygen gas becomes small.